The Filoviridae are non-segmented, single stranded RNA viruses which contain two divergent genera, Marburgvirus (MARV) and Ebolavirus (EBOV). Members from each can cause severe and highly lethal hemorrhagic fever disease to which there is no cure or licensed vaccine (Bradfute S. B., et al. (2011) Filovirus vaccines. Hum Vaccin 7: 701-711; Falzarano D., et al. (2011) Progress in filovirus vaccine development: evaluating the potential for clinical use. Expert Rev Vaccines 10: 63-77; Fields B. N., et al. (2007) Fields' virology. Philadelphia: Lippincott Williams & Wilkins 2 v. (xix, 3091, 1-3086 p.); Richardson J. S., et al. (2009) Enhanced protection against Ebola virus mediated by an improved adenovirus-based vaccine. PLoS One 4: e5308; and Towner J. S., et al. (2006) Marburgvirus genomics and association with a large hemorrhagic fever outbreak in Angola. J Virol 80: 6497-6516).
Due to lethality rates of up to 90% they have been described as “one of the most virulent viral diseases known to man” by the World Health Organization. The US Centers for Disease Control and Prevention has classified them as ‘Category A Bioterrorism Agents’ due in part to their potential threat to national security if weaponized (Burki T. K. (2011) USA focuses on Ebola vaccine but research gaps remain. Lancet 378: 389). These ‘high priority’ agents could in theory be easily transmitted, result in high mortality rates, cause major public health impact and panic, and require special action for public health preparedness (CDC (2011) Bioterrorism Agents/Diseases. Atlanta: Centers for Disease Control and Prevention).
The haemorrhagic fever diseases are acute infectious with no carrier state, although they are easily transmissible among humans and nonhuman primates by direct contact with contaminated bodily fluids, blood, and tissue (Feldmann H., et al. (2003) Ebola virus: from discovery to vaccine. Nat Rev Immunol 3: 677-685). During outbreak situations, reuse of medical equipment, health care facilities with limited resources, and untimely application of prevention measures escalate transmission of the disease, allowing amplification of infections in medical settings.
Since the natural reservoirs of these zoonotic pathogens are likely to be African bats and pigs (Kobinger G. P., et al. (2011) Replication, pathogenicity, shedding, and transmission of Zaire ebolavirus in pigs. J Infect Dis 204: 200-208), the latter possibly being more of an amplifying host, the manner in which the virus first appears at the start of an outbreak is thought to occur through human contact with an infected animal. Unpredictable endemic surfacing in the Philippines, potentially Europe, and primarily Africa of this disease further constitutes a major public health concern (Outbreak news. (2009) Ebola Reston in pigs and humans, Philippines. Wkly Epidemiol Rec 84: 49-50).
Experiments have been performed to determine the capacity of the vaccine for inducing protective efficacy and broad CTL including experiments in rodent preclinical studies. (Fenimore P W, et al. (2012). Designing and testing broadly-protective filoviral vaccines optimized for cytotoxic T-lymphocyte epitope coverage. PLoS ONE 7: e44769; Hensley L E, et al. (2010). Demonstration of cross-protective vaccine immunity against an emerging pathogenic Ebolavirus Species. PLoS Pathog 6: e1000904; Zahn R, et al (2012). Ad35 and ad26 vaccine vectors induce potent and cross-reactive antibody and T-cell responses to multiple filovirus species. PLoS ONE 7: e44115; Geisbert T W, Feldmann H (2011). Recombinant vesicular stomatitis virus-based vaccines against Ebola and Marburg virus infections. J Infect Dis 204 Suppl 3: S1075-1081; and Grant-Klein R J, Van Deusen N M, Badger C V, Hannaman D, Dupuy L C, Schmaljohn C S (2012). A multiagent filovirus DNA vaccine delivered by intramuscular electroporation completely protects mice from ebola and Marburg virus challenge. Hum Vaccin Immunother 8; Grant-Klein R J, Altamura L A, Schmaljohn C S (2011). Progress in recombinant DNA-derived vaccines for Lassa virus and filoviruses. Virus Res 162: 148-161).
Vaccine-induced adaptive immune responses have been described in numerous preclinical animal models (Blaney J E, et al. (2011). Inactivated or live-attenuated bivalent vaccines that confer protection against rabies and Ebola viruses. J Virol 85: 10605-10616; Dowling W, et al. (2007). Influences of glycosylation on antigenicity, immunogenicity, and protective efficacy of ebola virus GP DNA vaccines. J Virol 81: 1821-1837; Jones S M, et al. (2005). Live attenuated recombinant vaccine protects nonhuman primates against Ebola and Marburg viruses. Nat Med 11: 786-790; Kalina W V, Warfield K L, Olinger G G, Bavari S (2009). Discovery of common marburgvirus protective epitopes in a BALB/c mouse model. Virol J 6: 132; Kobinger G P, et al. (2006). Chimpanzee adenovirus vaccine protects against Zaire Ebola virus. Virology 346: 394-401; Olinger G G, et al. (2005). Protective cytotoxic T-cell responses induced by Venezuelan equine encephalitis virus replicons expressing Ebola virus proteins. J Virol 79: 14189-14196; Rao M, Bray M, Alving C R, Jahrling P, Matyas G R (2002). Induction of immune responses in mice and monkeys to Ebola virus after immunization with liposome-encapsulated irradiated Ebola virus: protection in mice requires CD4(+) T cells. J Virol 76: 9176-9185; Rao M, Matyas G R, Grieder F, Anderson K, Jahrling P B, Alving C R (1999). Cytotoxic T lymphocytes to Ebola Zaire virus are induced in mice by immunization with liposomes containing lipid A. Vaccine 17: 2991-2998; Richardson J S, et al. (2009) Enhanced protection against Ebola virus mediated by an improved adenovirus-based vaccine. PLoS One 4: e5308; Vanderzanden L, et al (1998). DNA vaccines expressing either the GP or NP genes of Ebola virus protect mice from lethal challenge. Virology 246: 134-144; Warfield K L, et al. (2005). Induction of humoral and CD8+ T cell responses are required for protection against lethal Ebola virus infection. J Immunol 175: 1184-1191; Jones S M, et al. (2007). Assessment of a vesicular stomatitis virus-based vaccine by use of the mouse model of Ebola virus hemorrhagic fever. J Infect Dis 196 Suppl2: S404-412 Grant-Klein R J, Van Deusen N M, Badger C V, Hannaman D, Dupuy L C, Schmaljohn C S (2012). A multiagent filovirus DNA vaccine delivered by intramuscular electroporation completely protects mice from ebola and Marburg virus challenge. Hum Vaccin Immunother 8; Geisbert T W, et al. (2010). Vector choice determines immunogenicity and potency of genetic vaccines against Angola Marburg virus in nonhuman primates. J Virol 84: 10386-10394.) Viral vaccines have shown promise and include mainly the recombinant adenoviruses and vesicular stomatitis viruses. Non-infectious strategies such as recombinant DNA and Ag-coupled virus-like particle (VLP) vaccines have also demonstrated levels of preclinical efficacy and are generally considered to be safer than virus-based platforms. Virus-specific Abs, when applied passively, can be protective when applied either before or immediately after infection (Gupta M, Mahanty S, Bray M, Ahmed R, Rollin P E (2001). Passive transfer of antibodies protects immunocompetent and immunodeficient mice against lethal Ebola virus infection without complete inhibition of viral replication. J Virol 75: 4649-4654; Marzi A, et al. (2012). Protective efficacy of neutralizing monoclonal antibodies in a nonhuman primate model of Ebola hemorrhagic fever. PLoS ONE 7: e36192; Parren P W, Geisbert T W, Maruyama T, Jahrling P B, Burton D R (2002). Pre- and postexposure prophylaxis of Ebola virus infection in an animal model by passive transfer of a neutralizing human antibody. J Virol 76: 6408-6412; Qiu X, et al. (2012). Ebola GP-Specific Monoclonal Antibodies Protect Mice and Guinea Pigs from Lethal Ebola Virus Infection. PLoSNegl Trop Dis 6: e1575; Wilson J A, et al. (2000). Epitopes involved in antibody-mediated protection from Ebola virus. Science 287: 1664-1666; Sullivan N J, et al. (2011). CD8(+) cellular immunity mediates rAd5 vaccine protection against Ebola virus infection of nonhuman primates. Nat Med 17: 1128-1131; Bradfute S B, Warfield K L, Bavari S (2008). Functional CD8+ T cell responses in lethal Ebola virus infection. J Immunol 180: 4058-4066; Warfield K L, Olinger G G (2011). Protective role of cytotoxic T lymphocytes in filovirus hemorrhagic fever. J Biomed Biotechnol 2011: 984241). T cells have also been shown to provide protection based on studies performed in knockout mice, depletion studies in NHPs, and murine adoptive transfer studies where efficacy was greatly associated with the lytic function of adoptively-transferred CD8+ T cells. However, little detailed analysis of this response as driven by a protective vaccine has been reported.
Countermeasure development will ultimately require an improved understanding of protective immune correlates and how they are modulated during infection. This proves difficult when infected individuals who succumb to filoviral disease fail to mount an early immune response. These fast-moving hemorrhagic fever diseases result in immune dysregulation, as demonstrated by the lack of a virus-specific Ab response and a great reduction in gross T cell numbers, leading to uncontrolled viral replication and multi-organ infection and failure. Conversely, survivors of Ebola virus (EBOV) disease exhibit an early and transient IgM response, which is quickly followed by increasing levels of virus-specific IgG and CTL. These observations suggest that humoral and cell-mediated immune responses play a role in conferring protection against disease. These data are also supported by numerous preclinical efficacy studies demonstrating the contribution of vaccine-induced adaptive immunity to protection against lethal challenge. However, mounting evidence has demonstrated a critical role for T cells in providing protection where efficacy was greatly associated with the functional phenotype of CD8+ T cells. While these recent studies highlight the importance of T cells in providing protection, their precise contributions remain uncharacterized and controversial. Furthermore, little detailed analysis of this response driven by a protective vaccine has been reported.